The rates of polymerization and depolymerization of identical concentrated deoxygenated hemoglobin S (HbS) solutions following a rapid temperature change were examined by several methods. Two of these methods measured viscosity changes in either gently agitated (AGT) or nonagitated (NAGT) samples. The third method utilized a change in turbidity at 735 nm (SDT). By all three methods, a delay period, during which no observable change was detected, followed the temperature change. Gelation, as determined in nonagitated samples by a viscosity- based technique, occurred before or coincided with gelation as determined spectrophotometrically. The slope of the concentration dependence of the delay time is significantly decreased by agitation. Similar monitoring of the depolymerization reaction indicated the persistence of increased viscosity after observation of a marked decrease in turbidity.